Compared to other styles of poetry under the category of Haiku and Eastern, there is comparatively little published about the more aggressive senryu.
Senryu, whose name means river willow, uses humor and satire to examine human society. Senryu takes on the form of haiku, but makes greater use of punctuation techniques (ellipses, exclamations, etc.) to convey its point. Senryu can use seasonal kigo, but do not rely on them. In senryu, the seasonal reference should be second in importance to the human portrayal. Contrary to popular belief, not all senryu is humorous. Many express misfortune, eroticism, political views (very important), religion & spirituality, and even anger (observational, not overflowing emotion like tanka). It is often bawdy, devoid of the subtle beauty known in haiku. Animals can also be represented through interaction using human personifications.
Originating during Japan's Edo period, senryu reflected both the societal and political turmoil of the time period. Popularized by a haijin named Senryu Karai, senryu was first recognized in the haiku contests going on in the cities. Haiku was written by the Japanese aristocratic class, court officials, and revered monks. Senryu was embraced and written by the common people. It was later given its own genre and studied alongside haiku and tanka.
While following the form of haiku, senryu is different in that it is not a form in itself. Senryu is a concept, a way of looking at things that are applied to haiku form, and a poetic genre that concerns human nature in its complex layers and emotions. More than being a style like haiku, known for its expression of nature within seasonal themes, senryu is a conceptual spinoff from haiku.
Since 2007, I have studied and written modern haiku, tanka, and senryu. To understand all three gives a wider range for expression. In the case of senryu, you become aware of your reactionary observations with the interaction of humanity from a social and cultural influence.
A small sampling of my own senryu offerings:
in my mailbox—
soccer ball out of bounds—
for the ice cream truck
the ciggies Dad left
ten years ago
so much mental baggage
a stick of gum in my pocket
beyond the reef
his son's trip to Peru